The Mechanism of Intrusion of the Inyo Dike, Long Valley Caldera, California

Abstract

We analyze the intrusion of the 11-km-long Inyo Dike at the margins of Long Valley caldera, eastem Califomia. The dike trends N07oW and is divided into at least three segments which are rotated by as much as 25 o with respect to the main trend. The dike seems affected primarily by the regional stress field of right-lateral shear of the western United States and by the local thermal conditions of the crest; the dike seems unaffected by the preexisting caldera margins and S ierra-Nevada frontal fault system. The high heat flow in Long Valley caldera implies that crustal rocks below 3-4.5 km deform by steady state creep under tectonic strain rate and support low to vanishing tectonic shear stresses. The upper rocks, above 3-4.5 km, deform by frictional slip along fractures and may support tectonic shear stresses as high as 24 MPa. We demonstrate that depth variations of tectonic stresses may have a profound effect on the segmentation and rotation of dikes, both at Long Valley and in other areas of high heat flow. The analysis places constraints on several tectonic conditions. The Inyo Dike intruded under a tectonic stress state with a horizontal maximum compression oriented N07oW. The maximum extensional fracture strength of the host rocks is 1-2.5 MPa, and the pressure drop within the propagating Inyo Dike was about 0.55 MPa/km. The volatile overpressure in the magma chamber was about 15 MPa during eruption of rhyolitic lavas at the Inyo Domes.